Due to recent increased energy demand, new gas and oil fields have been actively developed, and pipelines for transporting gas and oil have been increasingly installed in seismic zones and discontinuous permafrost zones. In seismic zones and discontinuous permafrost zones, however, the ground sometimes goes through large deformation due to liquefaction, fault displacement, and frost heaving and melting, or the like, thereby causing the pipelines buried in the ground to deform as well.
A plastically deformed pipeline buried in extremely deformed ground is also affected later on by large displacements. When an excessive displacement acts on the pipeline, a steel pipe making up the pipeline becomes bent, the compressive side of the pipeline buckles, and the buckled portion or the tensile side of the pipeline that is on the opposite side of the buckled portion fractures. To prevent damage to the steel pipe at the buckled portion and prevent gas or oil leakage from the buckled portion, high deformation performance is sought in steel pipes.
In the field of pipeline manufacture, main factors considered important in improving deformation performance of a steel pipe are steel material characteristics, particularly, the yield ratio of the steel material. Japanese Patent No. 4528356, Japanese Patent No. 4575995 and Japanese Patent No. 4575996 disclose techniques of manufacturing a steel pipe with improved deformation performance by reducing the axial yield ratio of a steel material of an electric resistance welded pipe with a composition and a dual-phase structure including a soft phase and a hard phase. In the technique of manufacturing a steel pipe with improved deformation performance disclosed in Japanese Patent No. 4442541, the yield ratio of a steel material of an electric resistance welded pipe is lowered by applying a strain in the plate-thickness direction and the length direction to the steel material during the process of incoming correction and rotation correction.
In the field of pipeline manufacture, it is also known to prevent buckling of the pipeline by identifying a region to which buckling countermeasures are to be provided, and increasing the stiffness near the region. In the technique disclosed in Japanese Patent Application Laid-open No. 2006-292088, because buckling near the welded portion leads to a tensile fracture, buckling near a welded portion is prevented by improving the local stiffness, by performing weld reinforcement to the ends of the steel pipe.
In relation to the shape of a UOE steel pipe, Japanese Patent Application Laid-open No. 2010-167440 discloses a technique that corrects the circularity of pipe ends using semi-circular upper and lower dies to prevent reduction in weldability due to mismatching of the shapes between the pipe ends to be welded in a centered arrangement with respect to each other. In the technique disclosed in Japanese Patent No. 3785998, a portion of a steel pipe not corrected with dies alone are pressed with rollers while rotating the steel pipe so that the circumferential irregularity in the circularity of the pipe ends resulted from the die shape are corrected and brought nearer to a true circle. The techniques disclosed in Japanese Patent Application Laid-open No. 2010-167440 and Japanese Patent No. 3785998 both ensure circularity of pipe ends by correcting the circumferential shape of the pipe ends.
A steel pipe structure such as a pipeline, a steel pipe pile, or a steel pipe sheet pile is a long structure formed by welding together a plurality of steel pipes in the longitudinal direction, each pipe of which has a length of approximately 12 meters to 24 meters. When an extensive deformation such as that of ground acts on such a steel pipe structure, the welded portion as well as the base material portion are both bent and deformed. It is generally known that the welded portion and the base material portion are different in stiffness because of the difference in the plate thickness resulting from weld reinforcement or the like or overmatching of a welding material, and that a pipe buckles mainly near the welded portion, in the bending-buckling tests of steel pipes. This tendency suggests that the welded portion has a lower deformation performance than the base material portion, and there is a limitation in increasing the yield ratio of the base material portion. Hence, the techniques disclosed in Japanese Patent No. 4528356, Japanese Patent No. 4575995, Japanese Patent No. 4575996 and Japanese Patent No. 4442541 alone cannot achieve a certain improvement of the deformation performance of the welded portion, when under consideration is a pipeline, and not a single steel pipe.
The technique disclosed in Japanese Patent Application Laid-open No. 2006-292088 aims to prevent a tensile force from overcoming defects in the circumferential welded portion. To achieve this goal, that technique reinforces the stiffness of a certain section near the circumferential welded portion by providing weld reinforcement to the section with a welding machine so that the buckling is prevented only near the circumferential welded portion. Generally, when a steel pipe buckles, the tensile strain overcomes the resistance on the rear side of the buckled portion, and causes the portion to fracture. With the technique disclosed in Japanese Patent Application Laid-open No. 2006-292088, therefore, concentration of tensile strain on a circumferential welded portion can be prevented. While the disclosure in Japanese Patent Application Laid-open No. 2006-292088 enables buckling prevention through stiffness improvement, buckling still occurs in portions not provided with weld reinforcement, near where the weld reinforcement ends. Furthermore, a steel pipe structure is expected to buckle with a smaller deformation than that in a structure only with a base material portion, due to the difference in the stiffness between the base material portion and the circumferential welded portion. While the technique disclosed in Japanese Patent Application Laid-open No. 2006-292088 can achieve the goal of preventing the fracture of a circumferential welded portion, deformation performance of the entire steel pipe structure still remains low.
Furthermore, the techniques disclosed in Japanese Patent Application Laid-open No. 2010-167440 and Japanese Patent No. 3785998 are also intended to improve the ease of welding at a circumferential joint. With the techniques disclosed in Japanese Patent Application Laid-open No. 2010-167440 and Japanese Patent No. 3785998, while the shape of the steel pipe is mainly corrected in the circumferential direction so that the circularity is improved, these techniques do not directly contribute to the improvement in the axial shape of a pipe. To prevent buckling of a steel pipe, correcting an axial shape is important, as will be described later. The techniques disclosed in Japanese Patent Application Laid-open No. 2010-167440 and Japanese Patent No. 3785998, therefore, are considered incapable of preventing buckling of a structure.
It could therefore be helpful to provide a UOE steel pipe with which a structure with improved buckling performance can be formed, and to provide a structure with improved buckling performance.